docs: def reddening(self,extval):
          """Compute the reddening for the given extinction.
  
          .. math::
  
              A(V) = R(V) \\; \\times \\; E(B-V)
  
              \\textnormal{THRU} = 10^{-0.4 \\; A(V)}
  
          .. note::
  
              ``self.litref`` is passed into ``ans.citation``.
  
          Parameters
          ----------
          extval : float
              Value of :math:`E(B-V)` in magnitudes.
  
          Returns
          -------
          ans : `~pysynphot.spectrum.ArraySpectralElement`
              Extinction curve to apply to a source spectrum.
  
          """
          T = 10.0**(-0.4*extval*self.obscuration)
          ans = ExtinctionSpectralElement(wave=self.wave,
                                          waveunits=self.waveunits,
                                          throughput=T,
                                          name='%s(EBV=%g)'%(self.name, extval))
          ans.citation = self.litref
          return ans